Heat removal and temperature control has become a highly important issue with CubeSats (100 mm×100 mm×100 mm-scale) and small satellite platforms (1 m×1 m×1 m-scale). A significant use of such satellites includes Earth observation using optical sensors. With increasing power budgets on CubeSats, increasing radio power requirements, and higher data rates, optical sensors experience more heat, Signal-to-noise ratio (SNR) decreases, and the quality of observation is reduced. Thus, efficient cooling systems for satellite optical sensors are required. The tasks for such a system include moving tens of watts away from a heat source and cooling to cryogenic temperatures below 123K (−150° C. or −238° F.) in order to enhance the SNR of the optical sensors. Excess heat may be radiated into Space by radiation from black panels according to Stefan-Boltzmann law. However, prior art cooling systems encounter several technical problems include size constraints and vibration issues, and lack necessary efficiencies.
A first technical problem of creating a miniature cryocooling system for small satellites is that it must be very small. That is, they must be less than about 100 mm and have a height of less than about 40 mm, in order to fit into 100 mm frame of a CubeSat.
A second technical problem is that compressors in such miniature cooling system must produce as little vibration as possible, because vibration distorts the image of the optical sensors on small satellites.
A third technical problem is that the miniature cooling system must be very efficient, and not use more energy than it removes, because very little energy (e.g., from solar panels) is available for the cooling systems of small satellites, and as much of the available energy as possible is required to perform other useful functions of the satellite.
A traditional approach for cooling systems for satellites is passive cooling. Passive cooling systems consisting of heat sinks have proven to be effective in a range of approximately 258 K to 313 K for sun synchronous orbits. More sophisticated passive cooling systems with ethane circulation between the cold and hot radiators such as the thermal control system designed for the PRISMA satellite of Agenzia Spaziale Italiana is capable of removing about 4.8 W of heat and cooling to around 185K (−88° C. or −127° F.). However, the temperature of 185K is above the cryocooling range (e.g. below 123K), and therefore does not meet the requirement.
Thus, active systems with a compressor can be significantly more effective, because the compressor heats the refrigerant through compression and enhances radiation of heat through black panels. Specifically, the Stefan-Boltzmann law states that while the total energy radiated per unit surface area of a black body is linearly dependent on the surface area of the radiating panels, it depends on the fourth power of the black panels' thermodynamic temperature T. For example, an active system developed by Lockheed Martin demonstrated removal of 0.65 W and cooling to 150K, but nonetheless, still does not reach the cryocooling range of below 123K and does not remove the tens of watts needed for cryocooling, and therefore does not meet the noted requirements.
Active cryocooling systems such as those based on reciprocating compressors or Stirling engines do come closer to or meet cryocooling ranges, but they also produce too much vibration due to two pistons constantly moving in reciprocating motion, and because of this, such systems do not meet the noted requirements. An example of such a miniature cryogenic cooling system is disclosed in U.S. Pat. No. 4,479,358.
Other prior art such as Joule-Thomson cryocoolers are too large and heavy for small satellites. Examples of these include the 4.3 kg Oxford cryocooler employed on UARS. These systems do not meet the noted requirements.
Miniature twin-screw or turbo compressors can solve the technical problem of vibration because they operate with rotary motion and produce minimal vibration. But it is well-known that in miniature sizes, with lengths shorter than 50 mm and heights lower than 40 mm, oil-free twin-screw and turbo compressors do not provide high compression ratios over 1:1.5, and therefore the temperature of refrigerant does not significantly increase through compression. Thus, very little heat is transferred to black radiation panels and radiated into space. For this reason twin-screw or turbo compressors do not meet the noted requirements.
As such, there is a need for a miniature rotary compressor in an active cooling system for use on a satellite that operates with minimal vibration, provides high compression ratio of 1:2 to 1:20, elevates the temperature of a refrigerant fluid in order to effectively radiate excess energy into Space according to Stefan-Boltzmann law, and enables removing tens of watts away from a heat source and cooling to cryogenic temperatures below 123K (−150° C. or −238° F.).